The present invention relates to a cooking appliance such as a microwave oven or an electric oven for heating objects including food.
Conventionally, a microwave oven with a moisture sensor determines that the food is completely heated by detecting the amount of vapor generated from the heated food. The output from the moisture sensor increases as vapor is generated from the heated food. When the output reaches a specified value (detection point), an additional heating time required for completing the food is calculated on the basis of a constant stored in an LSI control circuit. The oven then continues heating the food for the calculated period of time and then stops heating so that the most optimally heated food can be obtained. The time constant is different for different foods. For a certain kind of food, the user may be required to open the oven door in the middle of the heating process and to reverse and/or change the position of the food for more uniform heating. Usually, this intermediate food handling operation is carried out when the sensor output reaches the detection point level. For some foods, however, the intermediate food handling operation may be necessary at an earlier time. Frozen Hamburger Patties which are among the list of foods to be cooked by a sensor-equipped oven is an example of a food that needs to be reversed and/or moved in the middle of cooking so as to be uniformly heated. The sensor output increase for this food is, however, very slow. If the food is heated until the output reaches the detection point, therefore, it may be overheated locally, depending upon the quantity. If the food is reversed and/or moved at this stage, optimally heated food cannot be expected. That is, depending upon the quantity, the food may be required to be reversed and/or moved earlier than the detection point. One to three frozen hamburger patties can be optimally cooked if they are reversed and/or moved at the detection point. Four to six hamburger patties could be overheated locally if they were not moved until the detection point; they must be moved earlier than the detection point.
FIG. 2 shows the relationship between sensor output and heating time for four or more hamburger patties which are supposed to be heated each in a case.
Here, the detection point level of sensor output is considered to be 10 bits. The oven is designed to carry out additional heating for a specific time period after the sensor output reaches 10 bits. When the oven door is opened to take out the hamburger patties and reverse and/or change the positions in the middle of heating, part of the vapor generated from the food and accumulated within the heating chamber flows out of the oven, so that the output of the detector sensor drops. It begins rising again when the food is returned and heated again in the oven. Therefore, if this intermediate food handling operation is conducted before the sensor output reaches the detection point, the time required for the output to reach the detection point is a little longer than that in the case where the oven door is not opened until the detection point is reached. An arithmetic operation for calculating the additional heating time required after the detection point is reached takes account of this time lag.
If this intermediate food handling operation is carried out before the detection point is reached, in the oven instead of outside the oven, vapor and heat accumulated within the case are released all at once into the heating chamber when the case covers are opened. The vapor thus released partly flows into the exhaust duct leading to the detector sensor which is installed immediately above the heating chamber. Moreover, the magnetron cooling fan which generates an air stream in the heating chamber stops, when the door is opened, causing the vapor to stay in a part of the exhaust duct.
If the door is then closed and the heater is actuated with this state, the magnetron is energized and the magnetron cooling fan starts operating. This generates air current which sends the vapor in the exhaust duct to the detector sensor. When the sensor output at the intermediate food handling operation time (TA) is close to the detection point as shown in FIG. 2, the sensor output will reach the detection point immediately after the heating is resumed. As a result, the oven will be turned off before the food is sufficiently heated.